1. Field of the Invention
The present invention relates to a method for data communication of signals in units of a frame, using an orthogonal frequency division multiplexing (OFDM) algorithm, and in particular, to an initial frame synchronization method using the orthogonal frequency division multiplexing of signals for making it possible to find and synchronize the initial frame synchronization offset at the receiver in the frequency domain by an estimation of the variation of phase difference and the maximum likelihood (ML).
2. Description of the Conventional Art
Currently, there is increasing interest in multi-carrier modulation (MCM) for dividing a communication channel into several subchannels and transmitting may subcarriers through a single signal band using frequency division multiplexing (FDM) techniques. In MCM method, however, because several subcarriers occupying a narrow frequency domain are transmitted at one time, it results in a relatively longer symbol period compared with a single carrier modulation method, while MCM method has, owing to such characteristics, the advantages that equalization is easily performed and it has immunity to impulse noise.
The orthogonal frequency division multiplexing method which is a type of the above-described multi-carrier modulation, is a method to maximize the working frequency efficiency by securing an orthogonality among the multiplexed subcarriers. This multi-carrier modulation method, called "OFDM", has recently been applied by Cimini and Casas to the mobile radio channel with a multipath fading characteristic.(see.: L. J. Cimini, Jr., "Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing", IEEE Trans. Commun., vol. 33, No. 7, July 1985, pp. 665-675; and E. F. Casas, and C. Leung, "OFDM for Data Communication over Mobile Radio FM Channel--Part I: Analysis and Experimental Result", IEEE Trans. Commun., vol. 39, No. 5, May 1991, pp. 783-793).
FIG. 1 shows a block diagram of a general implementation of a transmitting and receiving system using the orthogonal frequency division multiplexing method. The orthogonal frequency division multiplexing modulator 10 consists, as shown in FIG. 1, of a serial/parallel converter 11 for converting input serial data into parallel data, an encoder 12 for encoding the output signals of the serial/parallel converter 11, an inverse fast Fourier transform (IFFT) converter 13 implementing the inverse fast Fourier transform of the output signals from the encoder 12, a parallel/serial converter 14 for converting the output signals of the inverse fast Fourier converter 13 into analog signals, and a digital/analog converter 15 converting the output data of the parallel/serial converter 14 into analog signals, while the orthogonal frequency division multiplexing demodulator 20 comprises an analog/digital converter 21 inputting the output signals of the orthogonal frequency division multiplexing modulator 10 through a transmitter 31, channels for transmission 32 and a receiver 33, and converting them into digital signals, a serial/parallel converter 22 converting the output signals of the analog/digital converter 21 into parallel data, a fast Fourier transform converter 23 for implementing a fast Fourier transform of the output signals of the serial/parallel converter 22, a decoder 24 for decoding the output signals of the fast Fourier converter 23, and a parallel/serial converter 25 for converting the parallel data output by the decoder 24 into serial data.
Therefore, in the orthogonal frequency division multiplexing transmitting/receiving system, the modulation and demodulation of the parallel data are carried out using the fast Fourier transform, and it is required to separate again the sampled data into a predetermined frame unit, and pass it through the fast Fourier transform routine, to restore at the receiver the data which was transmitted after being modulated by the frame unit. However, if the synchronization is in error in the course of forming the frame, the signals demodulated after the fast Fourier transform will be influenced by inter channel and intersymbol interference. Accordingly, the problem of synchronization in forming the frame must be treated especially as a matter of importance.
As an existing method for frame synchronization, there has used the method of detecting the energy of the signals received then performing coarse synchronization using a correlation, and then realizing a fine synchronization all in the time domain. (3. W. D. Warner, C. Leung, "OFDM/FM frame synchronization for mobile radio data communication", IEEE Trans. Veh. Technol. vol. 42, No. 3, August 1993, pp. 302-313).
In such conventional synchronization method as above-described, problems arise in that the process of synchronization is not only very complex, but the synchronization is not realized rapidly.